This chapter provides preliminary remarks and experimental methods on introducing electronic structure of semiconductor surface. Surface states are expected to occur near forbidden energy regions of the bulk band structure, particularly inside hybridization gaps or in the proximity of band edges. The determination of the surface band structure for the low index surfaces of semiconductors has been one of the major achievements of the surface physics in the last twenty years. In these materials the highly covalent character of the chemical bond and the presence of band gaps throughout the whole Brillouin zone make the surface spectra particularly rich of structures. To a large extent the progress in this field has been made possible by two major experimental
developments: (i) the availability of angle-resolved spectroscopies with high energy resolution; (ii) the discovery of the scanning tunnelling microscope as a probe for both structural and electronic properties. The major role played by these two spectroscopical techniques is due to the following features: (i) they are able to probe both energy and momentum of the electrons; (ii) they measure absolute electron energies rather than differences between electronic energy levels; (iii) they are complementary to each other since ARUPS probes occupied states, while KRIPS provides information on the empty ones. Moreover it has been found that in both direct and inverse photoemission the spectra are dominated by one-electron features and the structures associated with multielectron excitations are comparatively weaker. A projection for the (110) face of a partially ionic semiconductor ZnSe is shown.